Lasers and Modern Warfare

Photonics is used heavily within military equipment andapplications vary from guidance systems, such as gyroscopes, to range findersand countermeasure devices. Laser-based weapons are also being developed andglobal security company Northrop Grumman, as part of the US military’sJoint High Power Solid State Laser programme, has produced a light ray createdby a solid-state laser measuring 105kW. This is a major milestone as 100kW istraditionally viewed as the power threshold for ‘weapons grade’ lasers althoughlower power systems can be used, depending on the application. The systemutilises laser amplifier chains, each producing 15kW of power output with highbeam quality, which allows the weapon to be scalable in power depending on thethreat. Seven laser chains were combined to produce the 105kW beam andpotential applications for the weapons system include force protection andprecision strike missions.

Military research, typically, is concerned with very highpower lasers, as, according to Graham Catley, EMEA sales director at Gooch andHousego, this increases the effective range of the devices. ‘In terms of usinghigh-power lasers for R&D in industrial applications, the maximum poweroutput tends to be in the tens or hundreds of watts range, whereas for militaryresearch kilowatt laser systems are used,’ he comments. Gooch and Housegoprovides optical components and sub-systems for a number of differentindustries including military and defence.

Beam combination

High power, however, is only one variable and often thewavelength of the beam is vital for certain applications. Pranalytica, acompany based in Santa Monica, California, producing quantumcascade lasers (QCL), is supplying its laser technology as part of a projectfor the US Army Aviation and Missile Command (AMCOM). The project, which beganin early January 2009 and which is led by the Defense Advanced ResearchProjects Agency (DARPA) Small Business Innovation Research (SBIR) programme, isdesigned to increase the power output of a laser source by combining the beamsof a number of high-power QCLs. The aim is to develop more effective lasersources in the mid- (MWIR) and long-wave infrared (LWIR) for directionalinfrared countermeasures (DIRCM), advanced stand-off chemical sensors used todetect explosives or chemical warfare agents (CWA), and laser radar (LADAR).

Phase I of the project looks at combining an array of 200mWaverage power, thermoelectrically cooled QCLs into a 1W module. Pranalytica’sQCL modules, which in themselves were developed under the EfficientMid-Infrared Laser (EMIL) programme from DARPA, can produce more than 2W at4.6μm (MWIR) from a single emitter. Therefore, the power output required tosatisfy Phase I of the beam combining project could be generated using one QCLsystem. However, the project aims to increase the power output, with Phase IIextending the approach to produce more than 5W in the MWIR, and Phase IIItaking that further to exceed 10W.

Two watts of output power is high for a single QCL systemand while certain military research programmes are using kilowatt lasers, thewavelength requirements for these applications are very specific and are met byQCL modules. Dr C Kumar Patel, president and CEO of Pranalytica, explains thatQCLs are beneficial for DIRCM and explosives detection (either in situ orremote), because the laser output wavelengths can be tailored for the specificneeds. DIRCM applications require wavelengths in the 4-5μm MWIR region, whilefor explosives detection, wavelengths in the 8-12μm LWIR region are optimal.

‘Five QCL modules could potentially be combined to produce10W of power, however, the challenge is not only to combine the beams, but tomaintain the beam quality,’ explains Patel. Individual emitters produce a beamthat is close to a Gaussian profile and as the beams are combined this profilewould have to be maintained to allow the beam to be effective over long distances.‘The ultimate goal for the project is to combine any number of laser sources,not simply five, to get an arbitrarily high power output while maintaining beamquality.’

Missile deactivation

The high power output would increase the capabilities of DIRCMsystems, advanced stand-off chemical sensors and LADAR systems, used by the US military.DIRCM systems are designed to protect military aircraft from heat-seekingmissiles by deactivating their guidance systems. The missile locks onto theaircraft’s heat signature from its exhausts, which is at an effectivetemperature of approximately 700°K and has its peak emission wavelength in theMWIR region, and hones in on this to target the aircraft. As a countermeasure,a DIRCM device is attached to the underside of the aircraft and is primed torecognise a missile. The device then directs a MWIR laser beam at the missileof the same wavelength range onto which the missile locks. The beam output is aseries of pulses designed to disable the missile’s guidance system causing itto lose its target.

‘Currently, several aerospace and defence companies areusing 2W lasers in DIRCM systems, but the higher the power output the greaterthe effective range of the device and the further from the aircraft the missilecan be deactivated,’ says Patel.

QCL systems, operating in the 8-12μm wavelength region, havealso been used to provide accurate local detection of explosive and chemicalwarfare agents. Pranalytica’s L-PAS (laser photoacoustic) sensor, beingdeveloped under DARPA’s L-PAS project and that uses LWIR QCLs, to detect andidentify CWAs and explosives by analysing air samples drawn into the sensor. Afurther DARPA programme is being conducted using CO2 lasers for explosivedetection, which are expected to provide a range of approximately 200m. QCLmodules provide greater laser wavelength tuneability than CO2 lasers, whichallows the system to detect a wider range of explosives. However, to match thestand-off distance of 200m, the QCLs will need to match the power output of theCO2 lasers, which is approximately 5W. Patel explains that by combining outputpower from a number of QCLs, each tuneable in wavelength, the project aims toprovide a powerful and flexible system that can be tuned to identify specificexplosive substances at large stand-off distances.

The military also use laser technology to illuminatespecific targets. Laser designators shine a series of coded pulses onto atarget, which scatter into the atmosphere and are picked up by the seeker of aguided missile. These laser pulses guide the missile onto the target.

Target designators operate over several kilometres and emita beam that is in either in the MWIR or LWIR. The regions of 4-5μm and 8-12μmare critical because, at these wavelengths, there is relatively low opticalabsorption arising from carbon dioxide and water vapour. Therefore, theseregions, which are known as the first and the second infrared windows, allowlong distance propagation of the beam through the atmosphere.

Pranalytica’s PoyntIR device emits at wavelengths of 4.6μmor 9.6μm and can be used in battle zones to illuminate targets for ground-basedtroops and mobile combat vehicles at distances greater than 1km and for combataircraft at up to 10km. Lasers can also be used for so-called IFF (identifyfriend or foe) applications, whereby, as well as target designation, thetechnology can summon help or alert friendly forces to the soldier’s location.

There are numerous other areas within the military anddefence sector where photonics plays a role. Catley of Gooch and Housego notesthat communication channels within military vehicles and equipment are beingupgraded to fibre optic methods of delivery. ‘In place of copper wiring as thetraditional communication medium, photonics is being adapted as the technologyof choice,’ he says. ‘The cabling enabling a control panel to talk to a lasersystem when modulating the beam output is now typically fibre optic, andprojects such as FONDA (Future Optical Network Distribution for Aerospace) havebeen instrumental in developing fibre optic communication networks foraerospace and defence products.’ Fibre optic cabling provides a high bandwidthfor fast data transport and has the security benefit of the data beingencrypted when sent.